In external drum image recording devices, a movable optical carriage is used to displace an image exposing or recording source in a slow scan direction while a cylindrical drum supporting recording material on an external surface thereof is rotated with respect to the image exposing source. The drum rotation causes the recording material to advance past the exposing source along a direction which is substantially perpendicular to the slow scan direction. The recording material is therefore advanced past the exposing source by the rotating drum in a fast scan direction.
An image exposing source may include an optical system for scanning one or more exposing or recording beams. Each recording beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded.
The recording media to be imaged by an external drum imaging system is commonly supplied in discrete sheets and may comprise a plurality of plates, hereinafter collectively referred to as xe2x80x9cplatesxe2x80x9d or xe2x80x9cprinting plates.xe2x80x9d Each plate may comprise one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet. Other layers may include one or more image recording (i.e., xe2x80x9cimageablexe2x80x9d) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates which are supported by a polyester support are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9xe2x80x3xc3x9712xe2x80x3, or smaller, to 58xe2x80x3xc3x9780xe2x80x3, or larger. The printing plate may additionally comprise a flexographic printing plate.
In accordance with embodiments of the recording device or imaging system described herein, it is desirable to center-justify the printing plate on the drum, for example, to reduce vibrations as the drum rotates. Counterweights can be positioned on each end of the drum to compensate for the extra weight of the plate to balance the drum to minimize or eliminate the introduction of vibration-induced artifacts into the images recorded on the plate.
In an external drum imaging system, a plate locating mechanism and method for operation of same are provided for automatically locating printing plates of various sizes in a staging position so as to be mounted on a predetermined position on external drum of the imaging system. The predetermined position in a particular embodiment is substantially center-justified on the external drum to help prevent artifacts in the recorded image.